Firearm ammunition feeding devices typically may be categorized as one that “pushes” cartridges, one that “pulls” cartridges, or one that operates with a combination of pulling and pushing cartridges. In the “push” category, are ammunition magazines with a spring-biased follower behind a row of cartridges held in the magazine and delivered for chambering by the firearm. For rimless cartridges, the number of rounds that can be fed this way is relatively unlimited. In contrast, only a limited number of rounds of rimmed cartridges (usually .22 or .17 caliber rimfire or shotgun shells) can be magazine-fed this way because of the enlarged rim diameter relative to the body of the cartridge and because they must be fed in a way that the rims do not block feeding of an adjacent cartridge.
A “pull” type ammunition feeding device typically includes belt-fed mechanisms or a rotary magazine in which cartridges are engaged and moved by sprocket teeth. Some ammunition feeding mechanisms use a combination of “pull” and “push,” in which cartridges are conveyed by a belt or a sprocket until reaching a point at which a limited number of rounds are “pushed” into position to be chambered.
Detachable ammunition magazines generally are either a box or a drum. To increase the capacity of a box magazine, it must be extended in length and/or hold cartridges in multiple rows (such as a “double-stack”). For rimmed cartridges, this presents certain challenges, since they are generally “pushed” by a follower in a box magazine. I addressed these problems for rimmed shotgun cartridges in my prior U.S. Pat. No. 8,448,364 issued May 28, 2013. To increase the capacity of a drum magazine, it must be made a larger diameter or must hold cartridges in multiple spiral or concentric rows. Drum magazines for rimmed shotgun shells have been made with a single row of cartridges held in a sprocket that “pulls” the shells, except for about two that are “pushed” into the short neck to the mouth of the magazine that attaches to the firearm. For example, see U.S. Pat. No. 4,487,103 issued Dec. 11, 1984. Single-row drum magazines for rimmed shotgun shells have been made in different diameters to hold 10, 12, 20, and even 25 rounds. Single-row rotary magazines have been made for rimfire cartridges, such as is shown in U.S. Pat. No. 3,239,959 issued Mar. 15, 1966.
Higher capacity drum magazines for rimless cartridges have been made with multiple rows carried in a spiral path, such as that shown in U.S. Pat. No. 4,745,842 issued May 24, 1988. They have also been made with multiple concentric rows handled by multiple concentric sprockets, such as those shown in U.S. Pat. No. 2,321,720 issued Jun. 15, 1943, 4,384,508 issued May 24, 1983, and U.S. Pat. No. 8,037,800 issued Oct. 18, 2011. The contents of the above-referenced patents are fully incorporated herein by reference. In these concentric designs, the rimless cartridges are fed first from an outer sprocket. Then, the outer sprocket stops rotating while a rotating inner sprocket feeds cartridges through a passageway in the outer sprocket. Such a “pass-through” design requires the cartridges of either inner sprocket to be “pushed” a longer distance than those fed from the outer sprocket. This is not a problem for rimless rifle and pistol cartridges. However, due to multiple challenges and problems associated with “pushing” rimmed cartridges and transitioning them from one handling system to another, there has not been a drum magazine for rimmed cartridges held in multiple concentric rows where they pass through an outer sprocket from an inner sprocket.